Sea Level Rise Projections and Policy

One blog I follow is TrustYetVerify. The latest post – Projecting sea level 300, nah, 1000 years in the future – is straightforward and highlights some significant issues for climate policy.

He compares claims of an activist in a Belgium newspaper that unmitigated climate change will result in sea level rise of 5 metres in 300 years, with a graphic from UNIPCC AR5 WG1 Chapter 13 on sea level rise that showed a at most around a 3 metre rise.

There was a good spot by Michel in relation to a graphic from a December 2017 presentation on the impacts of an 8 metre rise in sea levels by the year 3000. In was originally from a 2004 Greenpeace document. Only the earlier document also had the impacts of current sea inundation and a 1 metre sea level rise.

There are some lessons that can be learnt.

Marginal Difference of policy

The current sea coverage is of large areas of the Netherlands that are not currently covered by sea water. To create the graphic, they have removed the dykes that have enabled the Netherlands to vastly increase its land area. This not only vastly exaggerates the impact of sea level rise, but contains the assumption that people are too dumb to counter the impact of sea level rise by building dykes higher. Given that even the exaggerated claims are 5 metres in 300 years, that means an average rate of rising of 17mm per annum and a maximum rate of maybe 30mm. What is more, any rise is predictable over maybe decades. Decisions can be made over 20-50 year timescales, which are far less onerous than taking the long-term perspective. Even if a 5 metre rise over 300 years was accurate, either building dykes now assuming sea levels are 5 metres higher, or abandoning areas that will be inundated will cause needless costs for this generation and the next few generations.
The is an even greater policy assumption, that I repeatedly point out. Climate mitigation through reducing greenhouse gas emissions requires that global emissions are reduced.  It does not matter whether Belgium, and the Netherlands make massive cuts their emissions, if most other countries do not follow similar policies. As a graphic 3.1 from the UNEP Emissions Gap Report 2017 clearly demonstrates, the net impact of all proposed policies is very little compared to doing nothing, and a long way from the 1.5°C or the 2°C targets. This is after over 20 years of annual COP meetings to obtain much bigger reductions.

The marginal impact of sea-level rise is therefore exaggerated by

  • Assuming that the existing flood defences vanish.
  • Assuming people do not build any more defences.
  • Exaggerating the projected rise.
  • Looking at a far greater timescale than rational planning ought to take place.
  • Falsely promoting emissions reductions to combat sea level rise impacts, knowing that whatever a few countries do will not make a difference to overall emissions. If significant warming is caused by human GHG emissions, and this leads to significant sea level rise, then current emissions policies are largely a waste of time.


Checking and Interpreting Forecasts / Projections

Consider the sea level rise graphic from UNIPCC AR5 WG1 Chapter 13 .

Consider the projections for the year 2500.

The High Scenarios shows sea level rise of 1.5 to 6.5m in 2500 for >700ppm CO2.
Medium scenarios show sea level rise of 0.2 to 2.3m in 2500 for 500-700ppm CO2.
Low scenarios show sea level rise of 0.5 to 1.0m in 2500 for <500ppm CO2.

How can the medium scenarios project a lower bottom end than the low scenarios?

The explanation probably lies in different modelling assumptions. After all the greater the scenario from the current state of affairs, the greater the uncertainty range, unless you assume that the structure of the model contains truths not revealing from any observations.

Further note the High scenarios lower limit is only 30cm a century, and the top end is 1.3m a century, whilst the medium scenarios bottom end over five centuries is roughly the rate of sea level rise per century for the last few centuries. That is, well within the medium scenario uncertainty range is the possibility that some global warming will make no difference to the rate of sea level rise.

What I also find interesting is that under the medium scenarios, Antarctica is gaining ice, hence reducing sea levels, but under the low scenarios has no impact whatsoever. Again, this shows the different modelling assumptions used.

Concluding note

Suppose a pharmaceutical company promoted a product with clearly exaggerated claims of its effectiveness, false alarm for the need for the product, and deliberately played down the harms that the product could cause to the patient? There would be an outcry, and the company being sued in a world without regulations. In most countries, strict regulations mean that to market a new product, the onus is on that company to demonstrate the product works, and that side effects are known. But it is alright to promote such falsehoods to “save the plant for future generations“. Indeed, to shout down critics as deniers of climate change. 

Kevin Marshall

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